As shown in FIG. 1, a cellular radio communication system mainly comprises terminals, base stations and a core network (CN). A radio access network (RAN) is a network formed by the base stations and responsible for the transaction of the access layer, for example, the management of the radio resources, etc. A physical or logical connection may exist among the base stations according to the actual conditions, for example, the connection between the base station 1 and the base station 2 or between the base station 1 and the base station 3 shown in FIG. 1. Each base station can be connected with one or more core network nodes. The core network is responsible for the transaction of a non-access layer, for example, the location updating, etc., and it is an anchor point of the user plane. The terminals refer to various devices for communicating with the cellular radio communication network, for example, mobile telephones or notebook computers, etc.
The cellular radio communication system takes a radio frame as the basic unit for identification of the system time, the serial number of the radio frame is known as radio frame number (SFN). The terminal can acquire the boundary of the radio frame through a cell search method, thereby achieving time synchronization on a downlink. In the cellular radio communication system such as the Long Term Evolution (LTE) system, the length of the radio frame is 10 ms (millisecond), and as shown in FIG. 2, as to the Frequency Division Duplex (FDD) mode, 10 subframes are included in one radio frame, the time length of each subframe is 1 ms, each subframe includes two time slots, and the time length of each time slot is 0.5 ms; as shown in FIG. 3, as to the Time Division Duplex (TDD) mode, generally speaking, one frame includes two half frames of which the time length is 5 ms, each half frame includes 5 subframes, the time length of each subframe is 1 ms, except for subframe #1 and subframe #6, other subframes also include two time slots, wherein the time length of the subframe and the time slot are the same with that in the FDD mode respectively; subframe #1 and subframe #6 respectively include 3 special time slots, i.e., DwPTS, GP and UpPTS. Subframe #6 can also be a normal subframe in some TDD frame formats, as shown in FIG. 4.
As to the LTE system, the length of a normal random access time slot is 1 ms, i.e., occupying the length of one subframe, and in addition, an extended random access time slot is also introduced, which may occupy the length of 2 or 3 subframes. A short random access time slot is also introduced in the TDD mode, i.e., transmitting on the UpPTS time slot.
As to the FDD mode, any subframe in time domain can be configured as a random is access time slot, but only one random access channel exists on one random access time slot. One random access channel occupies the bandwidth of 6 radio resource blocks (RB) in frequency domain. As to the TDD mode, subframe #0 and subframe #5 are always downlink time slots in time domain, therefore, subframe #0 and subframe #5 cannot be configured as random access time slots, the UpPTS time slots in subframe #1 and subframe #6 can be configured as random access time slots, whether other time slots except for the above can be configured as random access time slots is determined by the method of configuring an uplink time slot and a downlink time slot in the TDD frame structure, that the time slots configured as uplink subframes can be taken as the random access time slot. As to the TDD mode, one or more random access channels can exist on one random access time slot.
In the prior LTE system, the process of the terminal accessing into the cellular radio communication system comprises three steps as follows:
Step a, the terminal transmits a random access preamble message to the base station though a certain random access time slot of the radio frame;
Step b, the base station responds the terminal with a random access response message;
Step c, the terminal determines whether a correct response message is received according to the group identifier in the random access response message and the index of the random access preamble.
In step a, one or more terminals may transmit random access preamble messages to the base station though the same random access time slot, that these random access preamble messages may be the same with or different from each other, and the base station can identify the different random access preamble messages on the same random access time slot.
In step b, the random access response message may contain response information in response of one or more random access preamble messages. These random access preamble messages are all transmitted though the same random access time slot. Probably more than one random access response messages are merged into one response message, mainly in order to improve the utilization ratio of the radio resources in the random access process. In order to enable the terminal to identify the random access response messages, the base station adds a group identifier in the message, and there is a corresponding relationship between the group identifier and the random access time slot though which the random access preamble message is transmitted. Meanwhile, the random access response message may also contain an individual identifier corresponding to the random access preamble message itself, which is usually an index number of the random access preamble. The method for setting the group identifier is regulated in the protocol in advance, and when certain terminal transmits the random access preamble message to the base station, it has already been known what group identifier and individual identifier are to be received.
To ensure that the random access response process has certain flexibility, the random access response message is not synchronous with the random access preamble message, i.e., there is no fixed relationship between the two in time domain; on the contrast, the random access response message is allowed to be transmitted in a time window. At the same time, to increase the flexibility of the scheduling of the radio sources, to respond the random access preamble message received though certain random access time slot, the base station can transmit the random access response message corresponding to the random access preamble message on one or more Transmission Time Intervals (TTIs) in the time window. The start time of the time window is related to the speed of the base station to process the random access preamble message, and its end time is related to the load of the base station to process the random access preamble message and the radio resources scheduled to the random access response message, and other factors.
In step c, after the terminal receives one random access response message in the specified time window, firstly, the terminal verifies whether the expected group identifier is included in the message; if the expected group identifier is included in the message, the terminal then verifies whether the individual identifier (for example, index number) corresponding to the transmitted random access preamble message is also included in is the response message; if the expected individual identifier (for example, index number) is included, it can be determined that the current random access response message corresponds to the transmitted random access preamble message.
In the prior art, a method for setting the group identifier in step b is provided. Generally, in these methods a group identifier is calculated according to the absolute location of the random access time slot in the system time, therefore the group identifier is unique within the specified time range. These methods have the disadvantages that firstly, the terminal need to acquire the absolute system time of the cellular system in which the random access time slot locates, which generally refers to SFN, however, in real application, for example, the SFN of the target cellular system may be not known by the terminal in advance during the handover process, the group identifier cannot be calculated, and extra delay and system processing, for example, reading the system message, are usually needed to acquire the SFN of the target cellular system, because the SFN is usually broadcast in the system message.